Optoelectronic devices including heterojunction and intermediate layer

ABSTRACT

Embodiments generally relate to optoelectronic semiconductor devices such as solar cells. In one aspect, a device includes an absorber layer made of gallium arsenide (GaAs) and having only one type of doping. An emitter layer is located closer than the absorber layer to a back side of the device and is made of a different material and having a higher bandgap than the absorber layer. A heterojunction is formed between the emitter layer and the absorber layer, and a p-n junction is formed between the emitter layer and the absorber layer and at least partially within the different material at a location offset from the heterojunction. An intermediate layer is located between the absorber layer and the emitter layer and provides the offset of the p-n junction from the heterojunction, and includes a graded layer and an ungraded back window layer.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a Continuation application of U.S. patentapplication Ser. No. 14/692,647, entitled “Optoelectronic DevicesIncluding Heterojunction and Intermediate Layer,” filed on Apr. 21,2015, which is a Continuation application of U.S. patent applicationSer. No. 13/451,455, entitled “Optoelectronic Devices IncludingHeterojunction and Intermediate Layer,” filed on Apr. 19, 2012, which isa Divisional application of U.S. patent application Ser. No. 12/939,077,entitled “Optoelectronic Devices Including Heterojunction,” filed onNov. 3, 2010, and U.S. patent application Ser. No. 13/451,455 is also aContinuation-in-Part of U.S. patent application Ser. No. 12/605,108,filed on Oct. 23, 2009, entitled “Photovoltaic Device,” which claimspriority to U.S. Provisional Application No. 61/107,954, filed on Oct.23, 2008. Each of these applications is incorporated herein by referencein its entirety.

BACKGROUND Field of the Invention

Embodiments of the invention generally relate to optoelectronicsemiconductor devices such as photovoltaic devices including solarcells, and methods for fabricating such optoelectronic devices.

Description of the Related Art

As fossil fuels are being depleted at ever-increasing rates, the needfor alternative energy sources is becoming more and more apparent.Energy derived from wind, from the sun, and from flowing water offerrenewable, environment-friendly alternatives to fossil fuels, such ascoal, oil, and natural gas. Being readily available almost anywhere onEarth, solar energy may someday be a viable alternative.

To harness energy from the sun, the junction of a solar cell absorbsphotons to produce electron-hole pairs, which are separated by theinternal electric field of the junction to generate a voltage, therebyconverting light energy to electric energy. The generated voltage can beincreased by connecting solar cells in series, and the current may beincreased by connecting solar cells in parallel. Solar cells may begrouped together on solar panels. An inverter may be coupled to severalsolar panels to convert DC power to AC power.

Nevertheless, the currently high cost of producing solar cells relativeto the low efficiency levels of contemporary devices is preventing solarcells from becoming a mainstream energy source and limiting theapplications to which solar cells may be suited. During conventionalfabrication processes for photovoltaic devices, metallic contacts areoften deposited with a vapor deposition process, and usually heated totemperatures of over 300° C. during thermal anneal processes. These hightemperature processes are generally expensive due to the excessiveconsumption of time and energy. Also, the high temperature processesoften damage sensitive materials contained within the photovoltaicdevice.

Accordingly, there is a need for optoelectronic devices with increasedefficiency and methods for fabricating such optoelectronic devices atreduced costs when compared to conventional solar cells.

SUMMARY

Embodiments generally relate to optoelectronic semiconductor devicesincluding photovoltaic cells and the fabrication processes for formingsuch devices.

In one embodiment, an optoelectronic semiconductor device includes anabsorber layer made of gallium arsenide (GaAs) and having one type ofdoping. An emitter layer is located closer than the absorber layer to aback side of the device, the emitter layer made of a different materialand having a higher bandgap than the absorber layer. A heterojunction isformed between the emitter layer and the absorber layer, and a p-njunction is formed between the emitter layer and the absorber layer at alocation offset from the heterojunction. The p-n junction causes avoltage to be generated in the device in response to the device beingexposed to light at a front side of the device. An intermediate layerlocated between the absorber layer and the emitter layer and providesthe offset of the p-n junction from the heterojunction. The intermediatelayer has the same type of doping as the absorber layer and includes agraded layer having a material gradation from GaAs at a side closer tothe absorber layer to the different material of the emitter layer, and aback window layer not having the gradation and having an approximatelyuniform composition of the different material.

In another embodiment, a solar cell includes an absorber layer made ofgallium arsenide (GaAs) and having one type of doping, and an emitterlayer made of a different material and having a higher bandgap than theabsorber layer. An intermediate layer is provided between the absorberlayer and the emitter layer, the intermediate layer having the same typeof doping as the absorber layer, wherein the intermediate layer includesa material gradation from GaAs at a side closer to the absorber layer,to the different material of the emitter layer at a side closer to theemitter layer. The intermediate layer includes a graded layer having thegradation, and a back window layer not having the gradation and havingan approximately uniform composition of the different material. Aheterojunction is formed between the emitter layer and the absorberlayer, and a p-n junction is formed between the emitter layer and theabsorber layer and at least partially within the different material at alocation offset from the heterojunction. The p-n junction causes avoltage to be generated in the cell in response to the device beingexposed to light at a front side of the solar cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B depict a cross-sectional view of a photovoltaic unit inaccordance with one embodiment described herein;

FIGS. 1C-1D depict cross-sectional views of a portion of thephotovoltaic units of FIGS. 1A-1B in accordance with differentembodiments described herein;

FIG. 2 depicts a cross-sectional view of a two-sided photovoltaic cellin accordance with some embodiments described herein; and

FIG. 3 depicts a cross-sectional view of a single-sided photovoltaiccell in accordance with other embodiments described herein.

DETAILED DESCRIPTION

Embodiments generally relate to optoelectronic semiconductor devices andprocesses including photovoltaic devices and processes, and morespecifically relate to photovoltaic cells and the fabrication processesfor forming such photovoltaic cells and metallic contacts. Some of thefabrication processes include epitaxially growing thin films of galliumarsenide materials which are further processed by an epitaxial lift off(ELO) process. Some embodiments of photovoltaic cells described hereinprovide a gallium arsenide based cell containing an n-type film stackdisposed over a p-type film stack, such that the n-type film stack isfacing the front or sun side while the p-type film stack is on the backside of the cell. In one embodiment, the photovoltaic cell is atwo-sided photovoltaic cell and has an n-metal contact disposed on thefront side while a p-metal contact is disposed on the back side of thecell. In another embodiment, the photovoltaic cell is a single-sidedphotovoltaic cell and has the n-metal and the p-metal contacts disposedon the back side of the cell. Embodiments of an optoelectronic deviceinclude an absorber layer, an emitter layer, a heterojunction formedbetween emitter and absorber layers, and a p-n junction between theemitter layer and the absorber layer and at a location offset from theheterojunction, as described in greater detail below. Describedinnovations may allow for greater efficiency and flexibility inphotovoltaic devices when compared to conventional solar cells.

Some embodiments provide processes for epitaxially growing Group III-Vmaterials at high growth rates of greater than 5 μm/hr, such as about 10μm/hr or greater, about 20 μm/hr or greater, about 30 μm/hr or greater,such as about 60 μm/hr or greater including about 100 μm/hr or greateror about 120 μm/hr or greater. The Group III-V materials are thin filmsof epitaxially grown layers which contain gallium arsenide, galliumaluminum arsenide, gallium aluminum indium phosphide, gallium aluminumphosphide, or combinations thereof.

FIG. 1A illustrates a cross-sectional view of a photovoltaic unit 90containing a gallium arsenide based cell 140 coupled with a growth wafer101 by a sacrificial layer 104 disposed therebetween. Multiple layers ofepitaxial materials containing varying compositions are deposited withinthe photovoltaic unit 90 including the buffer layer 102, the sacrificiallayer 104, as well as many of the layers contained within the galliumarsenide based cell 140. The various layers of epitaxial materials maybe grown or otherwise formed by deposition process such as a chemicalvapor deposition (CVD) process, a metal organic CVD (MOCVD) process, ora molecular beam epitaxy (MBE) process.

In another embodiment described herein, the photovoltaic unit 90 may beexposed to a wet etch solution in order to etch the sacrificial layer104 and to separate the gallium arsenide based cell 140 from the growthwafer 101 during an epitaxial lift off (ELO) process. The wet etchsolution generally contains hydrofluoric acid, and may also containvarious additives, buffers, and/or surfactants. The wet etch solutionselectively etches the sacrificial layer 104 while preserving thegallium arsenide based cell 140 and the growth wafer 101. Onceseparated, the gallium arsenide based cell 140, as depicted in FIG. 1B,may be further processed to form a variety of photovoltaic devices,including photovoltaic cells and modules, as described by severalembodiments herein.

The Group III-V materials are thin films of epitaxially grown layerswhich may contain gallium arsenide, gallium aluminum arsenide, amongothers. Some layers, such as the window layer may contain additionalmaterials including gallium aluminum indium phosphide, gallium aluminumphosphide, or combinations thereof. The epitaxially grown layers may beformed by growing Group III-V materials during a high growth rate vapordeposition process. The high growth rate deposition process allows forgrowth rates of greater than 5 μm/hr, such as about 10 μm/hr or greater,about 20 μm/hr or greater, about 30 μm/hr or greater, such as about 60μm/hr or greater including about 100 μm/hr or greater or about 120 μm/hror greater as compared to the conventional observed deposition rates ofless than 5 μm/hr.

The process includes heating a wafer to a deposition temperature ofabout 550° C. or greater, within a processing system, exposing the waferto a deposition gas containing a chemical precursor, such as galliumprecursor gas and arsine for a gallium arsenide deposition process, anddepositing a layer containing gallium arsenide on the wafer. The highgrowth rate deposition process may be utilized to deposit a variety ofmaterials, including gallium arsenide, aluminum gallium arsenide,aluminum gallium phosphide, aluminum gallium indium phosphide, aluminumindium phosphide, alloys thereof, dopant variants thereof, orcombinations thereof. In some embodiments of the deposition process, thedeposition temperature may be within a range from about 550° C. to about900° C. In other examples, the deposition temperature may be within arange from about 600° C. to about 800° C. In other examples, thedeposition temperature may be within a range from about 650° C. to about750° C. In other examples, the deposition temperature may be within arange from about 650° C. to about 720° C.

In one embodiment, a deposition gas may be formed by combining or mixingtwo, three, or more chemical precursors within a gas manifold prior toentering or passing through the showerhead. In another embodiment, thedeposition gas may be formed by combining or mixing two, three, or morechemical precursors within a reaction zone after passing through theshowerhead. The deposition gas may also contain one, two or more carriergases, which may also be combined or mixed with the precursor gasesprior to or subsequent to passing through the showerhead.

The deposition gas may contain one or multiple chemical precursors ofgallium, aluminum, indium, arsenic, phosphorus, or others. Thedeposition gas may contain a gallium precursor gas which is an alkylgallium compound, such as trimethylgallium or triethylgallium. Thedeposition gas may further contain an aluminum precursor gas which is analkyl aluminum compound, such as trimethylaluminum or triethylaluminum.The deposition gas may further contain an indium precursor gas which isan alkyl indium compound, such as trimethylindium.

In some embodiments, the deposition gas further contains a carrier gas.The carrier gas may contain hydrogen (H₂), nitrogen (N₂), a mixture ofhydrogen and nitrogen, argon, helium, or combinations thereof. In manyexamples, the carrier gas contains hydrogen, nitrogen, or a mixture ofhydrogen and nitrogen. Each of the deposition gases may be provided tothe processing chamber at a flow rate from about 5 sccm (standard cubiccentimeters per minute) to about 300 sccm. The carrier gases may beprovided to the processing chamber at a flow rate from about 500 sccm toabout 3,000 sccm.

In other embodiments, the deposition gas contains the arsine and thegallium precursor gas at an arsine/gallium precursor ratio of about 3 orgreater, or may be about 4 or greater, or may be about 5 or greater, ormay be about 6 or greater, or may be about 7 or greater. In someexamples, the arsine/gallium precursor ratio may be within a range fromabout 5 to about 10. In other embodiments, the Group III-V materials maybe formed or grown from a deposition gas containing a ratio of Group Vprecursor to Group III precursor of about 30:1, or 40:1, or 50:1, or60:1, or greater. In some examples, the deposition gas has aphosphine/Group III precursor of about 50:1.

The processing system may have an internal pressure within a range fromabout 20 Torr to about 1,000 Torr. In some embodiments, the internalpressure may be ambient or greater than ambient, such as within a rangefrom about 760 Torr to about 1,000 Torr. In some examples, the internalpressure may be within a range from about 800 Torr to about 1,000 Torr.In other examples, the internal pressure is within a range from about780 Torr to about 900 Torr, such as from about 800 Torr to about 850Torr. In other embodiments, the internal pressure may be ambient or lessthan ambient, such as within a range from about 20 Torr to about 760Torr, preferably, from about 50 Torr to about 450 Torr, and morepreferably, from about 100 Torr to about 250 Torr.

The deposition processes for depositing or forming Group III-Vmaterials, as described herein, may be conducted in a single waferdeposition chamber, a multi-wafer deposition chamber, a stationarydeposition chamber, or a continuous feed deposition chamber. Onecontinuous feed deposition chamber that may be utilized for growing,depositing, or otherwise forming Group III-V materials is described inthe commonly assigned U.S. Ser. Nos. 12/475,131 and 12/475,169, bothfiled on May 29, 2009, which are herein incorporated by reference.

In one embodiment, one or more buffer layers 102 may be formed on thegrowth wafer 101 in order to start forming the photovoltaic unit 90. Thegrowth wafer 101 may contain an n-type or semi-insulating material, andmay contain the same or similar material as the one or more subsequentlydeposited buffer layers. For example, the growth wafer 101 may containgallium arsenide, or n-doped gallium arsenide, when creating a galliumarsenide, or n-doped gallium arsenide, buffer layer. The p-dopants maybe selected from carbon, magnesium, zinc, or combinations thereof whilethe n-dopants may be selected from silicon, selenium, tellurium, orcombinations thereof. In some embodiments, p-type dopant precursors mayinclude carbon tetrabromide (CBr₄) for a carbon dopant,bis(cyclopentadienyl)magnesium (Cp₂Mg) for a magnesium dopant, anddialkyl zinc compounds including dimethylzinc or diethylzinc for a zincdopant. In other embodiments, n-type dopant precursors may includesilane (SiH₄) or disilane (Si₂H₆) for a silicon dopant, hydrogenselenide (H₂Se) for a selenium dopant, and dialkyl tellurium compoundsincluding dimethyltellurium, diethyltellurium, and diisopropyltelluriumfor a tellurium dopant.

The buffer layer 102 or layers may provide an intermediary between thegrowth wafer 101 and the semiconductor layers of the final photovoltaicunit that can accommodate their different crystallographic structures asthe various epitaxial layers are formed. The one or more buffer layers102 may be deposited to a thickness from about 100 nm to about 600 nm,such as a thickness of about 500 nm, for example. Each of the one ormore buffer layers 102 may contain a Group III-V compound semiconductor,such as gallium arsenide, depending on the desired composition of thefinal photovoltaic unit. The buffer layer 102 may also be doped, such asan n-doped material, for example n-doped gallium arsenide.

A sacrificial layer 104 may be deposited on the buffer layer 102. Thesacrificial layer 104 may contain a suitable material, such as aluminumarsenide or an aluminum arsenide alloy, and may be deposited to have athickness within a range from about 3 nm to about 20 nm, such as fromabout 5 nm to about 10 nm, for example, about 10 nm. The sacrificiallayer 104 may also be doped, such as an n-doped material, for examplen-doped aluminum arsenide. The sacrificial layer 104, also known as therelease layer, is etched and removed while separating the galliumarsenide based cell 140 from the growth wafer 101 during the ELOprocess. Prior to being etched, the sacrificial layer 104 is alsoutilized to form the lattice structure for the subsequently andepitaxially grown layers contained within the gallium arsenide basedcell 140, such as the n-type contact layer 105.

The gallium arsenide based cell 140 includes an n-type film stack 120containing n-doped gallium arsenide materials disposed over a p-typefilm stack 130 which contain p-doped gallium arsenide materials. Each ofthe n-type film stack 120 and the p-type film stack 130 independentlycontains multiple layers of varying compositions of materials includinggallium arsenide materials. In one embodiment, the n-type film stack 120includes an n-type contact layer 105, an n-type front window 106, ann-type absorber layer 108 formed adjacent the n-type front window 106,and optionally, an intermediate layer 114. The p-type film stack 130includes a p-type emitter layer 110 and a p-type contact layer 112formed on the p-type emitter layer 110.

During a fabrication process, as described in one embodiment, the n-typecontact layer 105, or interface layer, may be deposited on thesacrificial layer 104. The n-type contact layer 105 contains Group III-Vmaterials, such as gallium arsenide, depending on the desiredcomposition of the final photovoltaic unit. The n-type contact layer 105is n-doped, and for some embodiments, the doping concentration may bewithin a range greater than about 1×10¹⁸ atoms/cm³, such as greater thanto 6×10¹⁸ atoms/cm³, for example, from greater than about 1×10¹⁸atoms/cm³ to about 1×10¹⁹ atoms/cm³. The n-type contact layer 105 may beformed at a thickness within a range from about 10 nm to about 1,000 nmor from about 10 nm to about 100 nm, such as from about 25 nm to about75 nm, for example, about 50 nm. The n-type contact layer 105 may beformed at this stage, such as a part of the gallium arsenide based cell140 prior to the ELO process. Alternatively, in another embodiment, then-type contact layer 105 may be formed at a later stage subsequent tothe ELO process. One advantage to forming the n-type contact layer 105as a part of the gallium arsenide based cell 140 prior to the ELOprocess is that the n-type contact layer 105 helps to protect the n-typefront window 106 from undesired damage or material contamination duringsubsequent processing steps, such as while etching the sacrificial layer104 during the ELO process.

An n-type front window 106, also known as a passivation layer, may beformed on the sacrificial layer 104, or if present, on the optionalcontact layer 105. The n-type front window 106 may contain a Group III-Vmaterial such as aluminum gallium, aluminum gallium arsenide, alloysthereof, or combinations thereof. The n-type front window 106 materialmay be n-doped, and for some embodiments, the doping concentration maybe within a range greater than about 1×10¹⁸ atoms/cm³, such as greaterthan to 3×10¹⁸ atoms/cm³, for example, from greater than about 1×10¹⁸atoms/cm³ to about 1×10¹⁹ atoms/cm³. The n-type front window 106material may be non-doped. The aluminum gallium arsenide may have theformula of molar ratios, the Al_(x)Ga_(1-x)As, for example, a molarratio of Al_(0.3)Ga_(0.7)As. The n-type front window 106 may bedeposited to have a thickness within a range from about 5 nm to about 75nm, for example, about 30 nm or about 40 nm. The n-type front window 106may be transparent to allow photons to pass through the n-type frontwindow 106 on the front side of the gallium arsenide based cell 140 toother underlying layers.

Alternatively, the n-type front window 106 may contain a material suchas aluminum gallium phosphide, aluminum gallium indium phosphide, alloysthereof, derivatives thereof, or combinations thereof. These aluminumgallium phosphide compounds provide for a large band gap, such as about2.2 eV, as well as high collector efficiency at lower wavelengths whenutilized within the n-type front window 106.

An absorber layer 108 may be formed on the front window 106. Theabsorber layer 108 may contain a Group III-V compound semiconductor,such as gallium arsenide. The absorber layer 108 may be monocrystalline.The absorber layer 108 may, for example, have only one type of doping,for example, n-doping, and for some embodiments, the dopingconcentration of the n-type absorber layer 108 may be within a rangefrom about 1×10¹⁶ atoms/cm³ to about 1×10¹⁹ atoms/cm³, for example,about 1×10¹⁷ atoms/cm³. The thickness of the n-type absorber layer 108may be within a range from about 300 nm to about 3,500 nm, such as fromabout 1,000 nm to about 2,000 nm (about 1.0 μm to about 2.0 μm), forexample, 2,000 nm.

As illustrated in FIG. 1B, an emitter layer 110, also referred to insome embodiments as a back window, may be formed adjacent the absorberlayer 108. The emitter layer 110 may, for example, be p-doped. Thep-type emitter layer 110 may contain a Group III-V compoundsemiconductor for forming a heterojunction with the n-type absorberlayer 108. For example, if the n-type absorber layer 108 containsgallium arsenide, the p-type emitter layer 110 may contain a differentsemiconductor material, such as aluminum gallium arsenide. If the p-typeemitter layer 110 and the n-type front window 106 both contain aluminumgallium arsenide, the Al_(x)Ga_(1-x)As composition of the p-type emitterlayer 110 may be the same as or different than the Al_(y)Ga_(1-y)Ascomposition of the n-type front window 106. For example, the p-typeemitter layer 110 may have a molar ratio of Al_(0.3)Ga_(0.7)As. Thep-type emitter layer 110 may be monocrystalline. The p-type emitterlayer 110 may be heavily p-doped and for some embodiments, the dopingconcentration of the p-doped emitter layer may be within a range fromabout 1×10¹⁷ atoms/cm³ to about 1×10²⁰ atoms/cm³, such as about 1×10¹⁹atoms/cm³. The thickness of the p-type emitter layer 110 may be within arange from about 100 nm to about 500 nm, for example, about 300 nm. Forsome embodiments, the n-type absorber layer 108 may have a thickness ofabout 800 nm or less, such as about 500 nm or less, such as within arange from about 100 nm to about 500 nm.

In some embodiments, the contact of the n-type absorber layer 108 withthe p-type emitter layer 110 creates a p-n interface layer for absorbingphotons. In embodiments in which the n-type absorber layer 108 containsone material (such as gallium arsenide) and the p-type emitter layer 110contains a different material having a different bandgap than thematerial of the absorber layer 108 (such as aluminum gallium arsenide),the p-n interface layer is a heterojunction. Heterojunctions, asdescribed in embodiments herein, are observed to have reduced darkcurrent, improved voltage productions, and improved radiativerecombinations as compared to homojunctions of the conventionalphotovoltaic materials. In some embodiments described herein, thematerial of the p-type emitter layer 110 has a higher bandgap than thematerial of the n-type absorber layer 108.

When light is absorbed near the p-n interface layer to produceelectron-hole pairs, the built-in electric field caused by the p-njunction may force the holes to the p-doped side and the electrons tothe n-doped side. This displacement of free charges results in a voltagedifference between the n-type absorber layer 108 and the p-type emitterlayer 110 such that electron current may flow when a load is connectedacross terminals coupled to these layers.

In some embodiments described herein, the p-type emitter layer 110 iscloser than the n-type absorber layer 108 to the back side of the cell140, i.e., the n-type absorber layer is closer to the front side of thecell 140. This arrangement of emitter layer under absorber layer can insome embodiments provide single carrier transport in the solar cell, inwhich the emitter and p-n junction are provided closer to the back sideof the cell such that the absorber layer absorbs most of the incidentphotons on the device and generates most of the carriers, such thatsubstantially a single type of carrier is generated. For example, withthe emitter layer 110 being made of a higher bandgap material than theabsorber layer, the emitter layer is more suited to absorb blue-spectrumphotons which do not penetrate as far into the device and thus not asmany of these photons reach the emitter layer that is further from thetop side than the absorber layer 108.

Fabricating a thinner base/absorber layer according to some embodimentsdescribed herein allows use of an n-doped base/absorber layer. Thehigher mobility of electrons in an n-doped layer compared to themobility of holes in a p-doped layer can lead to lower doping density inthe n-type absorber layer 108 as described by embodiments herein. Otherembodiments may use a p-doped base/absorber layer and an n-dopedback/emitter layer. For example, the base/absorber layer may be p-dopedin embodiments having a thicker absorber layer due to the diffusionlength of the carriers.

In other embodiments, as shown in FIG. 1B, an intermediate layer 114 maybe formed between the n-type absorber layer 108 and the p-type emitterlayer 110. The intermediate layer 114 can provide a material transitionbetween the n-type absorber layer 108 and the p-type emitter layer 110.

FIG. 1C shows a portion of one embodiment 150 of cell 140 includingabsorber layer 108, an intermediate layer 114, and emitter layer 110. Insome embodiments, the intermediate layer 114 contains the same orsubstantially the same material as the emitter layer 110, e.g., such asaluminum gallium arsenide in embodiments in which the emitter layer 110contains aluminum gallium arsenide. In addition, the intermediate layer114 has the same type of doping as the absorber layer 108. For example,the intermediate layer may have the formula of molar ratios ofAl_(x)Ga_(1-x)As, for example a molar ratio of Al_(0.3)Ga_(0.7)As, andbe n-doped within a range from about 1×10¹⁶ atoms/cm³ to about 1×10¹⁹atoms/cm³, for example 1×10¹⁷ atoms/cm³. The dopant concentrations canbe the same or substantially the same as the n-type absorber layer 108.In some embodiments the intermediate layer 114 can have a thickness ofabout two depletion lengths, where a depletion length is the width ofthe depletion region formed around the p-n junction. For example, insome embodiments the intermediate layer 114 can have a thickness in therange of about 0 to 200 nm.

This embodiment of the cell 140 provides a structure that allows the p-njunction that generates voltage for the cell to be offset from theheterojunction provided by materials having different bandgaps. Forexample, the p-n junction 152 is at the interface between the n-type andp-type materials of the emitter layer 110 and the intermediate layer114. Thus, in one described embodiment, the p-n junction is provided atleast partially within the higher-bandgap material of which the emitterlayer 110 and intermediate layer 114 are composed (e.g., AlGaAs), andthe heterojunction 122 is located at the interface between theintermediate layer 114 and the absorber layer 108 (e.g., the interfacebetween GaAs and AlGaAs). This offset provides some advantages over acoincident p-n junction and heterojunction. For example, the offset p-njunction provided between the AlGaAs layers can reduce barrier effectsof an interface between the AlGaAs and GaAs layers. In some embodiments,a majority of the absorber layer 108 is outside of a depletion regionformed by the p-n junction.

In some embodiments, the heterojunction 154 is located within twodepletion lengths of the p-n junction 152. For example, a depletionregion may be about 1000 Å (100 nm) wide in some embodiments. Thedepletion region typically still has a depletion effect past thisregion, within about two depletion region widths (depletion lengths) ofthe p-n junction. A heterojunction located further than this distancefrom the p-n junction may not allow the depletion effect to span theheterojunction interface and a barrier may thus exist.

As shown in FIG. 1D, in another embodiment 160 of the intermediate layer114, the intermediate layer 114 can contain a graded layer 115 and aback window layer 117 disposed between the absorber layer 108 and theemitter layer 110. For example, an n-type graded layer 115 can be formedover the n-type absorber layer 108 and an n-type back window 117 can beformed over the n-type graded layer 115, prior to forming the p typeemitter layer 110 over n-type back window 117. Each of the graded layer115 and the n-type back window 117 may be n-doped, and for someembodiments, the doping concentration may be within a range from about1×10¹⁶ atoms/cm³ to about 1×10¹⁹ atoms/cm³, for example 1×10¹⁷atoms/cm³, and the dopant concentrations are preferably the same orsubstantially the same as the n-type absorber layer 108. The thicknessesof the graded layer 115 and the back window 117 can vary widely indifferent embodiments, while the entire intermediate layer 114 canmaintain a standard thickness (e.g., about 2 depletion lengths, such asin the range of 0 to 200 nm in some embodiments). The back window 117can also provide passivation to reduce recombination at the surface ofthe absorber layer 108.

The embodiment of 160 includes a p-n junction 162 formed between then-doped layer 117 and the p-doped layer 110. The p-n junction 162 isoffset from the heterojunction 164 provided between two materials havingdifferent bandgaps. In the example of embodiment 160, the materials areGaAs in absorber layer 108 and AlGaAs in the graded layer 115. Althoughthe heterojunction 164 is shown in FIG. 1D for illustrative purposes ata midpoint in the graded layer, due to the material gradation theheterojunction may be at any point within the layer 115 or the entirewidth of the layer may be considered the heterojunction. As in theembodiment of FIG. 1C, the p-n junction is preferably offset from theheterojunction within two depletion lengths.

The graded layer 115 may be a graded layer that includes a materialgradation ranging from the absorber layer to the back window 117, wherethe gradation ranges from the material of the absorber layer at thegraded layer side closer to the absorber layer, to the material of theback window 117 at the side closer to the back window. Thus, using theexample materials described above, the gradation material may start asgallium arsenide adjacent the n-type absorber layer 108, and have agradation in the direction of the back window of an increasing amount ofaluminum and a decreasing amount of GaAs, such that the gradation endsadjacent the n-type back window 117 with about the same aluminum galliumarsenide material (molar ratios) as the material of back window 117. Inmany examples, the aluminum gallium arsenide at the window end of thegradation may have the formula of molar ratios, Al_(x)Ga_(1-x)As; forexample, a molar ratio of Al_(0.3)Ga_(0.7)As can be used. The gradationof the graded layer 115 may be parabolic, exponential or linear ingradation. The n-type back window 117 may also contain aluminum galliumarsenide and may have the formula of molar ratios, the Al_(x)Ga_(1-x)As,for example, a molar ratio of Al_(0.3)Ga_(0.7)As. In other embodiments,the intermediate layer 114 contains only the graded layer 115, or theintermediate layer 114 contains only the non-graded back window 117 (asshown in FIG. 1C).

Optionally, a p-type contact layer 112 may be formed on the p-typeemitter layer 110. The p-type contact layer 112 may contain a GroupIII-V compound semiconductor, such as gallium arsenide. The p-typecontact layer 112 is generally monocrystalline and p-doped, and for someembodiments, the doping concentration of the p-type contact layer 112may be greater than 1×10¹⁸ atoms/cm³, such as from about 6×10¹⁸atoms/cm³ to about 2×10¹⁹ atoms/cm³, for example, about 1×10¹⁹atoms/cm³. The p-type emitter layer 110 may have a thickness within arange from about 10 nm to about 100 nm, for example, about 50 nm.

Once the p-type emitter layer 110 has been formed, cavities or recesses(not shown) may be formed in the p-type emitter layer 110 (or optionalp-type contact layer 112) deep enough to reach the underlying basen-type absorber layer 108. Such recesses may be formed by applying amask to the p-type emitter layer 110 (or optional p-type contact layer112) using photolithography, for example, and removing the material inthe p-type emitter layer 110 (and optional p-type contact layer 112) notcovered by the mask using a technique, such as wet or dry etching. Inthis manner, the n-type absorber layer 108 may be accessed via the backside of the gallium arsenide based cell 140.

In other embodiments, the opposite type of doping can be used in thelayers discussed above, and/or other materials can be used that canprovide the described heterojunction and p-n junction. Furthermore, inother embodiments the layers can be deposited or formed in a differentorder than the order described above.

A photovoltaic unit created in this manner has a significantly thinabsorber layer, for example, less than 500 nm) compared to conventionalsolar units, which may be several micrometers thick. The thickness ofthe absorber layer is proportional to dark current levels in thephotovoltaic unit (e.g., the thinner the absorber layer, the lower thedark current). Dark current is the small electric current that flowsthrough the photovoltaic unit or other similar photosensitive device,for example, a photodiode, even when no photons are entering the device.This background current may be present as the result of thermionicemission or other effects. Because the open circuit voltage (V_(oc))increases as the dark current is decreased in a photosensitivesemiconductor device, a thinner absorber layer may most likely lead to agreater V_(oc) for a given light intensity and, thus, increasedefficiency. As long as the absorber layer is able to trap light, theefficiency increases as the thickness of the absorber layer isdecreased.

The thinness of the absorber layer may not only be limited by thecapabilities of thin film technology and ELO. For example, efficiencyincreases with the thinness of the absorber layer, but the absorberlayer should be thick enough to carry current. However, higher dopinglevels may allow current to flow, even in very thin absorber layers.Therefore, increased doping may be utilized to fabricate very thinabsorber layers with even greater efficiency. Conventional photovoltaicdevices may suffer from volume recombination effects, and therefore,such conventional devices do not employ high doping in the absorberlayer. The sheet resistance of the absorber layer may also be taken intoconsideration when determining the appropriate thickness.

Photovoltaic devices which contain a thin absorber layer as describedherein are usually more flexible than conventional solar cells having athickness of several micrometers. Also, the thin absorber layers asdescribed herein provide increased efficiency over conventional solarcells. Therefore, photovoltaic units according to embodiments may beappropriate for a greater number of applications than conventional solarcells.

FIG. 2 depicts one embodiment of a photovoltaic cell 200 which is atwo-sided photovoltaic device and therefore contains each of thecontacts, such as the p-metal contact layer 204 and the n-metal contactlayer 208, disposed on opposite sides of photovoltaic cell 200. Then-metal contact layer 208 is disposed on the front side or sun side toreceive light 210 while the p-metal contact layer 204 is disposed on theback side of photovoltaic cell 200. The photovoltaic cell 200 may beformed from the gallium arsenide based cell 140, as depicted in FIG. 1B,and as described by embodiments herein.

In one embodiment, an n-metal contact layer 208 is deposited on then-type contact layer 105 and subsequently, recesses are formed throughthe n-metal contact layer 208 and the n-type contact layer 105 to exposethe n-type front window 106 on the front side of the photovoltaic cell200. In an alternative embodiment, recesses may be initially formed inthe n-type contact layer 105 to expose the n-type front window 106 onthe front side of the photovoltaic cell 200. Thereafter, the n-metalcontact layer 208 may be formed on the remaining portions of the n-typecontact layer 105 while leaving exposed the n-type front window 106. Then-type contact layer 105 contains n-doped gallium arsenide materialswhich may have a dopant concentration of greater than about 3×10¹⁸atoms/cm³, such as within a range from greater than about 6×10¹⁸atoms/cm³ to about 1×10¹⁹ atom s/cm³.

An anti-reflective coating (ARC) layer 202 may be disposed over theexposed n-type front window 106, as well as the n-type contact layer 105and the n-metal contact layer 208, in accordance with an embodiment. TheARC layer 202 contains a material that allows light to pass throughwhile preventing light reflection from the surface of the ARC layer 202.For example, the ARC layer 202 may contain magnesium fluoride, zincsulfide, titanium oxide, silicon oxide, derivatives thereof, orcombination thereof. The ARC layer 202 may be applied to the n-typefront window 106 by a technique, such as sputtering. The ARC layer 202may have a thickness within a range from about 25 nm to about 200 nm,such as from about 50 nm to about 150 nm.

For some embodiments, the n-type front window 106, the p-type emitterlayer 110, and/or the p-type contact layer 112 may be roughened ortextured before applying the ARC layer 202. Each of the n-type frontwindow 106, the p-type emitter layer 110, and/or the p-type contactlayer 112 may be roughened by an etching process, such as a wet etchingprocess or a dry etching process. Texturing may be achieved by applyingsmall particles, such as polystyrene spheres, to the surface of then-type front window 106 before applying the ARC layer 202. By rougheningor texturing the n-type front window 106, the p-type emitter layer 110,and/or the p-type contact layer 112, different angles are provided atthe interface between the ARC layer 202 and the n-type front window 106,which may have different indices of refraction. In this manner, more ofthe incident photons may be transmitted into the n-type front window 106rather than reflected from the interface between the ARC layer 202 andthe n-type front window 106 because some angles of incidence for photonsare too high according to Snell's Law. Thus, roughening or texturing then-type front window 106, the p-type emitter layer 110, and/or the p-typecontact layer 112 may provide increased trapping of light.

In some embodiments, the n-type front window 106 may contain multiplewindow layers. For these embodiments, the outermost window layer (e.g.,the window layer closest to the front side of the photovoltaic cell 200)may be roughened or textured as described above before the ARC layer 202is applied, as illustrated in FIG. 2. In one embodiment, the n-typefront window 106 contains a first window layer (not shown) disposedadjacent to the n-type absorber layer 108 and a second window layer (notshown) interposed between the first window layer and the ARC layer 202.The first and second window layers may contain any material suitable forthe n-type front window 106 as described above, such as aluminum galliumarsenide, but typically with different compositions. For example, thefirst window layer may contain Al_(0.3)Ga_(0.7)As, and the second windowlayer may contain Al_(0.1)Ga_(0.9)As. Furthermore, some of the multiplewindow layers may be doped, while others are undoped for someembodiments. For example, the first window layer may be doped, and thesecond window layer may be undoped.

The p-metal contact layer 204 and/or the n-metal contact layer 208 eachcontain contact materials which are electrically conductive materials,such as metals or metal alloys. Preferably, the contact materialscontained within the p-metal contact layer 204 and/or the n-metalcontact layer 208 do not diffuse through other layers, such as asemiconductor layer, during any of the process steps utilized during thefabrication of the photovoltaic cell 200. Usually, each of the p-metalcontact layer 204 and the n-metal contact layer 208 contains multiplelayers of the same or different contact materials. The contact materialspreferably have specific contact resistance of 1×10⁻³ Ω-cm² or less.Preferred contact materials also have Schottky barrier heights (ϕ_(bn))of about 0.8 eV or greater at carrier concentrations of about 1×10¹⁸atoms/cm³. Suitable contact materials may include gold, copper, silver,aluminum, palladium, platinum, titanium, zirconium, nickel, chromium,tungsten, tantalum, ruthenium, zinc, germanium, palladium germaniumalloy, derivatives thereof, alloys thereof, or combinations thereof.

In some embodiments described herein, the p-metal contact layer 204and/or the n-metal contact layer 208 may be fabricated on thephotovoltaic cell 200 by a method, such as vacuum-evaporation through aphotoresist, photolithography, screen printing, or merely depositing onthe exposed surface of the photovoltaic cell 200 that have beenpartially covered with a resist mask, a wax, or another protectivematerial.

Optionally, a metal protective layer, or metal adhesion layer, may bedeposited on the p-metal contact layer 204. The metal protective layermay contain a material including nickel, chromium, titanium, alloysthereof, or combinations thereof. The metal protective layer preferablyexhibits good adhesion to p-doped gallium arsenide. In one exampleembodiment, the metal protective layer may be deposited to a thicknesswithin a range from about 5 Å to about 20 Å and have a reflectance ofabout 80% or greater. Preferably, the material of the metal protectivelayer and deposition thickness are deposited to minimize anyinterference with the reflectiveness of the p-metal contact layer 204.The metal protective layer may be deposited by an electron beamdeposition process or a PVD process, also known as a sputtering process.

Some example embodiments of p-metal contact layer 204, n-metal contactlayer 208, and other contact, adhesion, and reflector layers suitablefor use with contact layers of the cell 200 are described in copendingU.S. patent application Ser. No. 12/939,050, entitled, “MetallicContacts for Photovoltaic Devices and Low-Temperature FabricationProcesses Thereof,” filed on an even date herewith, and which isincorporated herein by reference. Other types, structures, and materialsof metal contact layers can also be used with cell 200.

FIG. 3 depicts a photovoltaic cell 300 which is a single-sidedphotovoltaic device and therefore contains both contacts, such as thep-metal contact 302 and the n-metal contact 312, disposed on the sameside of photovoltaic cell 300, as described by other embodiments herein.As shown in FIG. 3, both the p-metal contact 302 and the n-metal contact312 are on the back side of the photovoltaic cell 300 while the ARClayer 202 is on the sun side or front side of the photovoltaic cell 300that receives light 320. The p-metal contact 302 contains a p-metalcontact layer 304 disposed on a p-metal contact layer 306, while then-metal contact 312 contains an n-metal contact layer 308 disposed on ann-metal alloy contact 310, in some embodiments described herein.

In some embodiments, the photovoltaic cell 300 may be formed from thegallium arsenide based cell 140 of FIG. 1B. In one example, a resistmask may be formed on the exposed surface of the p-type contact layer112 and pattern recesses and holes may be formed during aphotolithography process. The pattern recesses and holes extend throughthe p-type contact layer 112, the p-type emitter layer 110, the n-typeback window 117, and the graded layer 115, and partially into the n-typeabsorber layer 108. Thereafter, the resist mask is removed to reveal then-type absorber layer 108 and the p-type contact layer 112 as theexposed surfaces on the back side of the photovoltaic cell 300, asviewed from the two-dimensional perspective towards the back side of thephotovoltaic cell 300. The sidewalls of the recesses and holes revealexposed surfaces of the p-type contact layer 112, the p-type emitterlayer 110, the n-type back window 117, and the graded layer 115, andpartially into the n-type absorber layer 108.

In one embodiment, the p-metal contact layer 306 is formed on a portionof the exposed the p-type contact layer 112 and the n-metal alloycontact 310 is formed on a portion of the exposed the n-type absorberlayer 108. Thereafter, the insulation layer 216 may be deposited overthe surface of the photovoltaic cell 300, such as to cover all exposedsurfaces including the p-metal contact layer 306 and the n-metal alloycontact 310. Subsequently, the exposed surfaces of the p-metal contactlayer 306 and the n-metal alloy contact 310 are revealed by etchingpattern holes into the insulation layer 216 by a lithography process. Insome embodiments, the p-metal contact layer 306 and the n-metal alloycontact 310 are formed prior to separating the gallium arsenide basedcell 140 from the growth wafer 101 during the ELO process while theinsulation layer 216 is formed subsequent to the ELO process. Thep-metal contact layer 304 may be formed on the p-metal contact layer 306and a portion of the insulation layer 216 while the n-metal contactlayer 308 may be formed on the n-metal alloy contact 310 and otherportions of the insulation layer 216 to form the photovoltaic cell 300,as depicted in FIG. 3. In some examples, the p-metal contact layer 304and the n-metal contact layer 308 may be formed containing the samecompositional layers of material as each other and in other examples,the p-metal contact layer 304 and the n-metal contact layer 308 aresimultaneously formed on the photovoltaic cell 300 during the samemetallization steps.

In an alternative embodiment, the p-metal contact 302 and the n-metalcontact 312 may be fabricated, in whole or in part, and subsequently,the insulation layer 216 may be formed over and on the sidewalls of therecesses between and around the p-metal contact 302 and the n-metalcontact 312. In another alternative embodiment, the insulation layer216, in whole or in part, may be formed on the photovoltaic cell 300prior to forming the p-metal contact 302 and the n-metal contact 312.

Despite all the contacts, such as the p-metal contact 302 and then-metal contact 312, being on the back side of the photovoltaic cell 300to reduce solar shadows, dark current and its stability with time andtemperature may still be concerns when designing an efficientphotovoltaic device, such as the photovoltaic cell 300. Therefore, forsome embodiments, an insulation layer 216 may be deposited or otherwiseformed on the back side of the photovoltaic cell 300. The insulationlayer 216 contains an electrically insulating material or grout whichhelps to reduce the dark current within the photovoltaic cell 300.

The insulation layer 216 may contain an electrically insulating materialor grout, such as silicon oxides, silicon dioxide, silicon oxynitride,silicon nitride, polysiloxane or silicone, sol-gel materials, titaniumoxide, tantalum oxide, zinc sulfide, derivatives thereof, orcombinations thereof. The insulation layer 216 may be formed by apassivation method, such as by a sputtering process, an evaporationprocess, a spin-coating process, or a CVD process.

In another embodiment, the insulation layer 216 eliminates orsubstantially reduces electrical shorts from occurring between thep-metal contact 302 and the n-metal contact 312. The insulation layer216 contains an electrically insulating grout and/or other electricallyinsulating material that has an electrical resistance of at least 0.5 MΩ(million ohms) or greater, such as within a range from about 1 MΩ toabout 5 MΩ, or greater. Exemplary grouts or other electricallyinsulating materials may contain a polymeric material, such as ethylenevinyl acetate (EVA), polyimide, polyurethane, derivatives thereof, orcombinations thereof. In one example, the electrically insulating groutcontains a photosensitive polyimide coating. In another example, theelectrically insulating grout contains a thermal set polymeric material.

In many embodiments, the n-metal alloy contact 310 may be formed by alow temperature process, which includes low temperature depositionprocesses followed by a low temperature, thermal anneal process.Suitable contact materials deposited within the n-metal alloy contact310 by low temperature deposition processes may include palladium,germanium, palladium germanium alloy, titanium, gold, nickel, silver,copper, platinum, alloys thereof, or combinations thereof, among others.

In another embodiment, the n-metal alloy contact 310 may containmultiple layers of conductive materials including a palladium germaniumalloy. The n-metal alloy contact 310 is disposed between the n-typeabsorber layer 108 and the n-metal contact layer 308 for providing astrong ohmic contact therebetween. The palladium germanium alloy withinthe n-metal alloy contact 310 allows a high conductivity of the electricpotential from the gallium arsenide materials within the n-type absorberlayer 108, across n-metal alloy contact 310, and to the n-metal contactlayer 308. The n-metal alloy contact 310 can also contain a metalliccapping layer which can be provided, for example, on the palladiumgermanium alloy layer. In some embodiments, the capping layer caninclude an adhesion layer and a high conductivity layer. For example,the adhesion layer can allow the conductivity layer to adhere to thealloy layer. In some examples, the adhesion layer may contain titanium,tin, zinc, alloys thereof, or combinations thereof and the highconductivity layer may contain gold, silver, nickel, copper, aluminum,alloys thereof, or combinations thereof, or a stack of multipledifferent metal layers and/or alloy layers. In one example, the n-metalalloy contact 310 contains a high conductivity layer containing golddisposed on an adhesion layer containing titanium, which is disposed ona palladium germanium alloy.

Similar fabrication methods and embodiments as described above for thep-metal contact layer 204 and/or the n-metal contact layer 208 on cell200 can be used for the p-metal contact layer 306 on photovoltaic cell300. Some example embodiments of n-metal alloy contact 304, p-metalcontact 302, n-metal contact 312, n-metal alloy contact 310, and otherlayers suitable for use with contact layers of the cell 300 aredescribed in copending patent application Ser. No. 12/939,050, entitled,“Metallic Contacts for Photovoltaic Devices and Low-TemperatureFabrication Processes Thereof,” filed on an even date herewith, andwhich is incorporated herein by reference. Other types, structures, andmaterials of metal contact layers can also be used with cell 300.

While the foregoing is directed to embodiments of the inventions, otherand further embodiments of the inventions may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

What is claimed is:
 1. An optoelectronic semiconductor devicecomprising: a base layer made of group III-V semiconductor material; anemitter layer made of a different material than the group III-Vsemiconductor material of the base layer and having a higher bandgapthan a bandgap of the base layer; a first junction formed at a locationbetween the emitter layer and the base layer; a second junction formedat a location between the emitter layer and the base layer offset fromthe first junction; a graded layer located between the emitter layer andthe base layer and having a material gradation from the group III-Vsemiconductor material of the base layer at a side closer to the baselayer, to the different material of the emitter layer; and a back windowlayer located between the emitter layer and the base layer and nothaving the material gradation and having an approximately uniformcomposition of the different material, the back window layer providingat least part of the offset between the first junction and the secondjunction, wherein contacts for the optoelectronic semiconductor deviceare on opposite sides of the optoelectronic semiconductor device.
 2. Theoptoelectronic semiconductor device of claim 1, wherein the graded layeris located adjacent to the base layer, and wherein the back window layeris located between the graded layer and the emitter layer.
 3. Theoptoelectronic semiconductor device of claim 1, wherein the secondjunction is located within two depletion lengths of the first junction.4. The optoelectronic semiconductor device of claim 1, wherein the groupIII-V semiconductor material of the base layer comprises galliumarsenide, aluminum gallium arsenide, aluminum gallium phosphide,aluminum gallium indium phosphide, aluminum indium phosphide, orcombinations thereof.
 5. The optoelectronic semiconductor device ofclaim 1, wherein the different material of the emitter layer comprisesgallium arsenide, aluminum gallium arsenide, aluminum gallium phosphide,aluminum gallium indium phosphide, aluminum indium phosphide, orcombinations thereof.
 6. The optoelectronic semiconductor device ofclaim 1, wherein the optoelectronic semiconductor device is a singlecarrier transport device.
 7. The optoelectronic semiconductor device ofclaim 1, wherein a majority of the base layer is outside of a depletionregion formed by the second junction.
 8. The optoelectronicsemiconductor device of claim 1, wherein the second junction causes avoltage to be generated in the optoelectronic semiconductor device inresponse to the optoelectronic semiconductor device being exposed tolight at a front side of the optoelectronic semiconductor device.
 9. Theoptoelectronic semiconductor device of claim 1, wherein the firstjunction is a heterojunction and the second junction is a p-n junction.10. A photovoltaic device comprising: a base layer made of group III-Vsemiconductor material; an emitter layer made of a different materialthan the group III-V semiconductor material of the base layer and havinga higher bandgap than a bandgap of the base layer; a graded layer havinga material gradation from the group III-V semiconductor material of thebase layer at a side closer to the base layer, to the different materialof the emitter layer at a side closer to the emitter layer; a backwindow layer not having the material gradation and having anapproximately uniform composition of the different material; a firstjunction formed at a location between the emitter layer and the baselayer; and a second junction formed at a location between the emitterlayer and the base layer and offset from the first junction, whereincontacts for the photovoltaic device are on opposite sides of thephotovoltaic device.
 11. The photovoltaic device of claim 10, whereinthe graded layer is located adjacent to the base layer, and wherein theback window layer is located between the graded layer and the emitterlayer.
 12. The photovoltaic device of claim 10, wherein the secondjunction is located within two depletion lengths of the first junction.13. The photovoltaic device of claim 10, wherein the group III-Vsemiconductor material of the base layer comprises gallium arsenide,aluminum gallium arsenide, aluminum gallium phosphide, aluminum galliumindium phosphide, aluminum indium phosphide, or combinations thereof.14. The photovoltaic device of claim 10, wherein the different materialof the emitter layer comprises gallium arsenide, aluminum galliumarsenide, aluminum gallium phosphide, aluminum gallium indium phosphide,aluminum indium phosphide, or combinations thereof.
 15. The photovoltaicdevice of claim 10, wherein the emitter layer is located closer than thebase layer to a back side of the photovoltaic device, such that singlecarrier transport is provided in the photovoltaic device.
 16. Thephotovoltaic device of claim 10, wherein a majority of the base layer isoutside of a depletion region formed by the second junction.
 17. Thephotovoltaic device of claim 10, where the second junction causes avoltage to be generated in the photovoltaic device in response to thephotovoltaic device being exposed to light at a front side of thephotovoltaic device.
 18. The photovoltaic device of claim 10, whereinthe first junction is a heterojunction and the second junction is a p-njunction.
 19. The photovoltaic device of claim 10, wherein the secondjunction formed at the location between the emitter layer and the baselayer is at least partially within the different material.
 20. Thephotovoltaic device of claim 10, wherein: the group III-V semiconductormaterial of the base layer is GaAs, with the GaAs of the based layerbeing n-type GaAs, the different material of the emitter layer and theback window layer is AlGaAs, with the AlGaAs of the emitter layer beingp-type AlGaAs and the AlGaAs of the back window layer being n-typeAlGaAs, and the material gradation of the graded layer is from GaAs toAlGaAs.